The present invention herein relates to a microfluidic driving system, and more particularly to a technology for fluid control by means of the microfluidic driving system.
Recently, the development of Micro Electro Mechanical Systems (MEMS) technology makes many large components become miniaturized. Within numerous research fields of the MEMS technology, what calls for particular attention is that microfluidic devices are applied to biomedical detections. The microfluidic biomedical detection chips produced by the MEMS technology not only have the advantages of high detection performance, low sample consumption, low energy, small size, and low production cost brought by the MEMS batch process, but also have the advantages of productions of low-cost disposable chips in order to reduce cross-contamination. Furthermore, with regards to a micro total analysis system (μ-TAS) having integrated micro fluidics, real-time response and simultaneous analysis, its development potential as well as its application value cannot be ignored. The production of the micro total analysis system will bring a great change in human life. The portable detection module of the system not only can be used on personal physical analysis anytime and anywhere, but also can be used on environmental detections, food testing, as well as various kinds of chemical analyses. The system is fast and time-saving, and it can be easily identified if only a small number of samples is required, which is quite environment-friendly.
For the micro total analysis system, a microfluidic driving system plays an indispensable component. Recently, the electroosmotic flow of the fluid driven by means of an induction electric field is highlighted for special attention by public research institutes due to no mechanical components being required to promote the fluid, which has a simple production and can be combined with the microfluidic systems easily. DC electroosmotic flows often require thousands of volts of high-voltage electric fields, and thus electrolytic reactions are easily produced to cause bubbles, which limit promotions and applications of the DC electroosmotic flows. AC electroosmotic flows generated by induced polarization charges have been proved to effectively avoid bubbles. This is because the AC electric field can control its frequency to be much greater than the inverse of the electrochemical time in order to improve such situation, and a lower voltage can be used.
The formation mechanism of the AC electroosmotic flow is similar to that of the DC electroosmotic flow, which also drives the fluid depending on Coulomb forces resulted from the electric filed acting on charges of the electric double layer. Smoluchowski equation of electrodynamics and viscous dissipation theory can be expressed by: Ut=−∈×ζ×Et/η, wherein “Ut” represents a tangential slip velocity of the electroosmotic flow, “∈” represents a dielectric constant of the electrolytic solution, “η” represents a viscosity coefficient of the electrode surface, “ζ” represents the surface potential, and “Et” represents the tangential component of an external electric field. However, for the AC electroosmotic flow, the charges of the electric double layer are no longer in Poisson-Boltzmann balance due to rapid charge-discharge processes, which causes an electrode polarization. For this reason, the polarization charges are gathered into the induced charges on the surface of the electrodes, as is like to charge the electric double layer capacitor with a non-uniform charge distribution. The condition that happens at high frequency and there is no electrochemical current flowing through the surface of the electrode is typically called as the capacitor charging.
Currently, researches for the AC electroosmotic flow are mainly focused on parallel electrodes. In addition, electrodes with different sizes will produce asymmetric electric fields to drive the fluid move toward the larger electrode. Please refer to FIG. 1. FIG. 1 is a diagram showing a large electrode 11 and a small electrode 12 disposed on the same plane according to the prior art. When an external AC electric field is added to the electrodes 11, 12, there is an electric field formed in the electrolytic solution 13, wherein a tangential component of the electric field acts on the polarization charges of the electrodes to make the polarization charges, affected by Coulomb forces, move outward the electrodes along the surface of the electrodes. Such effect can drive the fluid on the surface of the electrode to move outward the electrodes, and is called as an electroosmotic flow streamline direction 14. Furthermore, the electrolytic solution 13 located close to the electrode surface forms a pair of vortexes overturning from outside to inside the electrodes. Since the two electrodes 11 and 12 located on the same plane have different sizes, the electric field produced by the small electrode 12 is stronger so as to drive a streamline direction 15 of the electrolytic solution 13 to move toward the large electrode 11.
Additionally, since the hydrophobic film can reduce surface viscosity coefficients, and thus the boundary slip length can be increased to reduce flow resistance. Therefore, the electroosmotic flow effects depending on surface movements can be amplified to make the velocity of the electroosmotic flow of the hydrophobic surface increase more significantly than that of the hydrophilic surface, and the slip length between the Teflon film and the water is up to 100-200 nm. Moreover, if the frequency is too high, the strong polarization charges cannot be formed by the electric double layer to cause a flowing as the charges of the electrolytic solution does not have enough time to form a tight electric double layer. If the frequency is too low, the electric double layer performs a stronger screening effect upon the external electric field, and thus the tangential electric field won't be produced and its flow velocity is equal to zero. For this reason, the frequency should be close to an inverse of the RC time, which means the charge-discharge time of a circuit including a capacitor and a resistor or can be called as D/(λL), wherein D represents the ion diffusion coefficient, λ represents a thickness of the electric double layer, and L represents a separation distance between these two electrodes.
From the above, nonlinear AC electroosmotic flows resulted from 3D asymmetric electrodes capable of avoiding electrolysis and using hydrophobic surface will be provided in the present invention. As shown in FIG. 2, flow fields having horizontal or vertical vortexes are expected to be formed at the peripheries of each end of the electrodes.